1. Field of the Invention
The present invention relates to a light weight sheet molding compound.
2. Brief Description of the Prior Art
Recently, two-layered sheet molding compounds comprising a layer including reinforcing fiber and a layer including no reinforcing fiber have been used for an engine hood and outside plates of an automobile, for example, a rear spoiler, a roof, a hood and so on. These sheet molding compounds include a hollow filler in order to reduce the weight.
The light weight sheet molding compounds are produced as follows: Resin paste for compounds including a hollow filler is applied on two polyethylene films respectively at a predetermined thickness. Then, roving glass fiber is cut and scattered uniformly on one spread resin paste, thereby placing reinforcing fiber. Then, the other resin paste applied film is laminated thereon to make the laminated layers into one integral sheet. Therefore, as shown in FIG. 5, a conventional sheet molding compound 30 comprises an inner layer 31 including reinforcing fiber, and resin layers 32 including a hollow filler 33 and disposed on the both sides of the inner layer 31.
The viscosity of the sheet molding compound 30 varies with the heat for molding and hardening the compound 30. At that time, the filler 33 which has been dispersed in the resin paste and which has a smaller specific gravity than the base resin tends to move upward and concentrate on the surface. A part of the hollow filler 33 breaks or cracks by shearing force applied during molding the compound 30. These broken materials and the gas which has been contained in the filler 33 are scattered on the surface of the sheet molding compound 30. Therefore, the surface to be painted is not always smooth, and a paint film formed on the compound 30 often develops coating defects such as blisters. After-treatment such as repair of coating defects is required and productivity is decreased.
As an attempt to remove these disadvantages, Japanese Unexamined Patent Publication (KOKAI) No. 193940/1988 proposes a low specific gravity sheet molding compound in which one resin sheet layer in contact with the other resin sheet layer by way of reinforcing fiber includes no hollow filler, and a low specific gravity sheet molding compound in which one resin sheet layer includes a smaller amount of a hollow filler than the other resin sheet layer.
In the above sheet molding compounds in which the resin sheet layer on the surface side includes no hollow filler or a smaller amount of a hollow filler, however, only short fiber exists as reinforcing fiber between the one resin sheet layer and the other resin sheet layer including a larger amount of the hollow filler, and the filler can easily pass through the reinforcing fiber. Therefore, when the resin viscosity is lowered during heating for hardening the resin, it is difficult to inhibit the low specific gravity filler from passing through the reinforcing fiber and transferring to the surface side of the resin sheet layer. Further, it is sometimes hard to hold the resin layer including a smaller amount of the hollow filler on the upper side. Therefore, the sheet molding compound surface to be painted is difficult to be kept smooth, and appearance defects such as blisters are often developed in a paint film formed on the sheet molding compound surface. The coating defects necessitate repairing steps. Accordingly, productivity enhancement by removing steps of repairing the paint film is required to be used as panel component parts.